Why You Should Think Twice About Buying an Unbalanced miniDSP 2x4

Introduction

For several years now, people wanting to add one or more subwoofers to their audio or AV system, along with the ability to apply EQ and individual delay to the subs, have been asking about the miniDSP 2x4 DSP devices in online forums. They offer three different models of 2x4 device as of this writing: the standard 2x4 unbalanced, 2x4 balanced, and 2x4 HD versions. Unfortunately, the standard 2x4 unbalanced version, which appears at first blush to be the best choice for most users, has a problem that will prevent proper system operation in many, but not all cases.

The Problem: Insufficient Maximum Output Voltage For Many Systems

The standard unbalanced miniDSP 2x4 has a maximum output voltage of 0.9 Volts RMS. This is the maximum amplitude of a sinusoidal voltage it can supply at its output without significant distortion or clipping. Why does this matter? The answer lies in an important power amplifier specification called input sensitivity. This specification refers to the power amplifier connected to the miniDSP that's used to drive its associated subwoofer. The input sensitivity of a power amplifier is the amplitude of a sinusoidal input voltage required to drive the amplifier to its rated continuous power into a specified load. The "sensitivity" terminology is misleading, and might actually be considered a measuire of insensitivity, as the larger the "sensitivity" number is, the less sensitive the amp is to changes in input voltage. At any rate, the important thing to remember is that in order to be able to drive the amplifier to its rated output power, the maximum output voltage of the device driving the amplifier (in this case the miniDSP 2x4) must be greater than or equal to the amplifier's input sensitivity rating. For an exhaustive discussion of amplifier input sensitivity and its relationship to amplifier gain and output power, see Understanding Input Sensitivity Specs.

Things You Can Do to Ensure Success

• Check your sub amplifier's specification to make sure its input sensitivity is less than or equal to 0.9 Volts RMS.
• If you have a commercial sub with a built-in amplifier (an active sub), try to get the amp's input sensitivity specification from the sub's manufacturer.
• Commercial active subs that aren't of the professional audio variety tend to have amps with lower input sensitivity than those made for professional audio use. As such, they tend to be more compatible with the unbalanced miniDSP 2x4 than, for example, DIY subs using pro audio amplifiers. However, such subs rarely list the input sensitivity of their amps, and it can be nearly impossible to find this information. In such cases, though they may work properly with an unbalanced miniDSP 2x4, there is no guarantee that they will.
• The unbalanced 2x4 has an input sensitivity jumper that allows adjusting the maximum input voltage to either 0.9 Volts RMS or 2.0 Volts RMS. This is often confused with the maximum output voltage, which is 0.9 Volts RMS, regardless of the setting of the input sensitivity jumper. In fact, when the jumper is set for 2.0 Volts RMS maximum input, the device has a loss of about 7 dB. This input sensitivity jumper setting is almost always undesirable.
• Keep the power amplifier's input level control at or near its maximum value if you're using the unbalanced 2x4. If your amp has a 0.75 Volt input sensitivity and you reduce the input level by 6 dB, the input sensitivity is no longer 0.75 Volts, but 1.5 Volts.
• If your amp's input sensitivity is greater than 0.9 Volts RMS, use a balanced miniDSP 2x4 or miniDSP 2x4 HD instead.
• Even if the input sensitivity of your present amp is compatible with the output voltage of the unbalanced miniDSP 2x4, you might still want to use the balanced 2x4 or 2x4 HD anyway, in case you decide to update your system with a new amp in the future.
• Instructions for how to wire up the balanced 2x4 inputs and outputs can be found on pages 23-25 of the manual for the balanced 2x4. These instructions include balanced and unbalanced connections for input and output.
• If you don't like the idea of wiring up the Phoenix connectors of a balanced miniDSP 2x4, another alternative is the 2x4 HD version. This version costs about twice as much as the unbalanced 2x4. It has RCA connectors at input and output and a maximum output voltage of 2 Volts RMS, plenty for just about any amplifier. In addition, it has a very generous maximum per-sub delay of 80 msec. That's more than you'll ever need for any home application. By contrast, both non-HD 2x4 versions have a rather limited maximum delay of 7.5 msec when using the four-way advanced plugin, and 7.2 msec for the 2x4 advanced plugin. This may not be enough for some applications.
• There is one as-yet unknown aspect of the 2x4 HD device related to its use of a sample rate of 96 kHz rather than the 48 kHz sample rate of the other two 2x4 units. In the past, miniDSP units using a 96 kHz sample rate have shown inaccurate measured filter responses at very low frequencies despite the miniDSP software indicating virtually perfect peformance. This problem is due to the finite precision of the filter coefficients internal to the DSP chip. This is a common potential problem with all IIR filters. The workaround was to use a 48 kHz plugin instead. However, the original poster of that thread was still not completely satisfied with that solution. A detailed set of plots of this effect with the miniDSP 4x10 HD device, which has an Analog Devices DSP chip having the same filterr coefficient precision as the non-HD 2x4 and 2x4 balanced versions, can be found in a review of that device at the DIY Geezer site. It is located under "miniDSP 4x10 HD low frequency filter stability on 48 kHz plugin". Notice the increase in the peaking of the response at low frequencies as the cutoff frequency of the high-pass filter is decreased. Unlike the non-HD 2x4 versions, the 2x4 HD version uses a DSP chip having 32-bit floating-point coefficients. The non-HD 2x4 versions use a DSP chip with fixed-point filter coefficients. This use of floating-point filter coefficients may well be a mitigating factor for the 2x4 HD version. So far, it is unknown whether the change to 32-bit floating-point coefficients from a fixed-point configuration fixes the problem, given that there is also an increase in sample rate from 48 kHz to 96 kHz. Notice also that the "HD" designation in miniDSP products does not necessarily mean that the DSP chip has floating-point filter coefficients. The 4x10 HD has the same fixed-point coefficients as the non-HD 2x4 versions, and the same limited delay range as well.